var stream = require("stream");
var Readable = stream.Readable;
var rs = new Readable();
var c = 'a'.charCodeAt(0);
rs._read = function(){
    if(c<'z'.charCodeAt(0)){
        //添加数据
        rs.push(String.fromCharCode(c++));
    }else{
        rs.push(null);
    }
}
//将可读流输出一个可写流（pipe相当于一个管道，起连接作用）
rs.pipe(process.stdout);